Use eventlet.tpool.Proxy for DB API calls

There is no eventlet in common db code anymore, so, if we want to use
thread pooling for DB API calls, we should use eventlet.tpool.Proxy()
instead of tpool_wrapper() from openstack.common.db.api.

Config option CONF.database.use_tpool was moved from module
nova.openstack.common.db.api to nova.db.api.
Config file `nova.conf.sample` was updated as well.

See also patch to Oslo - I6fa140f2b1f03c43ae7cab98ac8a12d71188af76

Co-Authored-By: Chris Behrens <cbehrens@codestud.com>
Change-Id: Id7de85dce11f5ea61dfccddf80de7800a2bc4e37

nova/db/api.py

@@ -27,6 +27,7 @@ these objects be simple dictionaries.
 
 """
 
+from eventlet import tpool
 from oslo.config import cfg
 
 from nova.cells import rpcapi as cells_rpcapi
@@ -45,18 +46,54 @@ db_opts = [
     cfg.StrOpt('snapshot_name_template',
                default='snapshot-%s',
                help='Template string to be used to generate snapshot names'),
-    ]
+]
+
+tpool_opts = [
+    cfg.BoolOpt('use_tpool',
+                default=False,
+                deprecated_name='dbapi_use_tpool',
+                deprecated_group='DEFAULT',
+                help='Enable the experimental use of thread pooling for '
+                     'all DB API calls'),
+]
 
 CONF = cfg.CONF
 CONF.register_opts(db_opts)
+CONF.register_opts(tpool_opts, 'database')
 CONF.import_opt('backend', 'nova.openstack.common.db.options',
                 group='database')
 
 _BACKEND_MAPPING = {'sqlalchemy': 'nova.db.sqlalchemy.api'}
 
 
-IMPL = db_api.DBAPI(CONF.database.backend, backend_mapping=_BACKEND_MAPPING,
-                    lazy=True)
+class NovaDBAPI(object):
+    """Nova's DB API wrapper class.
+
+    This wraps the oslo DB API with an option to be able to use eventlet's
+    thread pooling. Since the CONF variable may not be loaded at the time
+    this class is instantiated, we must look at it on the first DB API call.
+    """
+
+    def __init__(self):
+        self.__db_api = None
+
+    @property
+    def _db_api(self):
+        if not self.__db_api:
+            nova_db_api = db_api.DBAPI(CONF.database.backend,
+                                       backend_mapping=_BACKEND_MAPPING)
+            if CONF.database.use_tpool:
+                self.__db_api = tpool.Proxy(nova_db_api)
+            else:
+                self.__db_api = nova_db_api
+        return self.__db_api
+
+    def __getattr__(self, key):
+        return getattr(self._db_api, key)
+
+
+IMPL = NovaDBAPI()
+
 LOG = logging.getLogger(__name__)
 
 # The maximum value a signed INT type may have